1. Field of the Invention
The present invention relates generally to epoxy resin composite materials. More particularly, the present invention is directed to uncured epoxy resin composite materials that may be stored over relatively long periods of time at room temperature.
2. Description of Related Art
Fibrous reinforced resin compositions are widely used as structural components, in industrial and leisure applications, in aerospace and for electronic circuit board applications. A multitude of various combinations of fibers and resins are presently in use. These fiber/resin compositions are generally referred to as xe2x80x9ccomposite materialsxe2x80x9d or simply xe2x80x9ccompositesxe2x80x9d. In many instances, the fibers and resin are combined in a suitable mold and immediately cured to form the finished product. Alternatively, the fibers and resin are combined with an appropriate curing agent and stored in an uncured state for future use. These types of pre-impregnated uncured composites are generally referred to as xe2x80x9cprepregxe2x80x9d.
Epoxy resins have been used as the resin matrix for a wide variety of composites. Epoxy resin prepreg typically includes a fibrous reinforcement that is embedded in an uncured resin matrix. The uncured resin matrix includes one or more liquid resins and one or more curing agents. The uncured resin matrix may also include additives such as tougheners, flame retardants, fillers and the like. Prepregs that include an epoxy matrix resin generally are cured at temperatures in the range from 50xc2x0 C. to 200xc2x0 C.
Epoxy prepregs have limited preservation stability at room temperature due to the presence of curing agent(s) in the resin matrix. This is particularly evident in low-temperature cure epoxy prepreg compositions. For example, a prepreg that includes an epoxy resin and an imidazole curing agent, that can be cured at 60xc2x0 C., can have a room temperature stability of less than two days rendering it unusable thereafter. Stability during storage at room temperature is important, especially if materials are to be transported long distances, and/or stored at ambient temperatures before use. It is, of course, possible to store these known prepregs at temperatures below room temperature to enhance their shelf life, but the need for large or expensive refrigeration units make this option undesirable.
Additionally it is essential that the matrix resin does not flow out of the fibrous reinforcement phase of the prepreg during storage or lay-up. This may be achieved by formulating solid epoxy resins, which have high molecular weights, or formulating highly viscous epoxy resins.
It is also desirable for at least one side of the prepreg to have a certain degree of tack. This is required during the lay up of prepreg materials. If the prepreg has no tack it can prove difficult to assemble together layers of prepreg in the mold.
It is, furthermore, desirable that the prepreg has a certain degree of flexibility to enable it to be formed into the desired shape. This is also required if the prepreg is to be supplied in rolls.
It is well known that epoxy resins may crystallize at room temperature but this is commonly regarded as a disadvantage and even a nuisance, because the crystallization can interfere with the mixing and other operations used in the manufacture of a conventional prepreg material.
According to a first aspect of the present invention there is provided an uncured composite material or prepreg that includes a fibrous reinforcement and a crystalline epoxy resin and a solid epoxy resin curing agent, wherein the epoxy resin curing agent is substantially uniformly dispersed in the crystalline epoxy resin. The uncured composite material includes an uncured composite body that has at least one side and is composed of fibers imbedded in a substantially crystallized solid matrix. The solid matrix includes a solids mixture of the crystallized epoxy resin and a solid curing agent that is preferably also crystallized.
The side of the uncured composite material has a certain level of tackiness and the uncured composite body has a certain degree of flexibility. In addition, a certain amount of the solid matrix flakes from the uncured composite body when it undergoes flexing. As a feature of the present invention, a property enhancement material is located within the composite body or as a layer located on the side of the uncured composite material. The enhancement agent is preferably an adhesive or flexibilizing agent. It was discovered that the use of an adhesive as the enhancement agent not only increases the tackiness of the uncured composite body, but also unexpectedly increases the flexibility of the body as well as reducing the amount of solid matrix that flakes from the body during flexing.
The epoxy resin prepreg compositions of the invention have excellent room temperature stability, typically of 3 to 6 months, thus providing increased room temperature shelf life. Furthermore they may be cured at a conveniently low temperature in the range from 50xc2x0 C. to 200xc2x0 C., and preferably at temperatures lower than 100xc2x0 C., and most preferably in the temperature range 40-70xc2x0 C. The use of epoxy resin that has been crystallized has no effect on the mechanical properties of the cured composite material compared with an epoxy resin that is not crystallized.
The above discussed and many other features and attendant advantages of the present invention will become better understood by reference to the detailed description when taken in conjunction with the accompanying drawings.